The interface shows the green "User added" message, but they never appear in the database - somewhere between submission and database insertion it's failing, but there's no easy path to find out where it's broken.

I'm at a bit of a loss. Any ideas?

Also, although not necessarily the issue, I have noticed a JavaScript bug in cms/javascript/MemberList.js on line 56:
$('Form_MemberForm').loadNewPage(response.responseText);

In both IE and Firefox, "$('Form_MemberForm')" is a non-existant object and which produces a JavaScript error in the browser.